An electrolytic copper foil is produced by using an aqueous solution composed of sulfuric acid and copper sulfate as an electrolyte, a titanium plate coated by iridium or an oxide thereof as a dimensionally stable anode (DSA), a titanium drum as a cathode, applying a direct current between two electrodes to electrodeposit copper ions in the electrolyte on the titanium drum, and then stripping the electrolytic copper foil from the surface of the titanium drum and continuously winding for manufacturing. The side that the electrolytic copper foil contacts with the surface of the titanium drum is referred to as “shiny side (S side),” and the back side of the electrolytic copper foil is referred to as “matte side (M side).” Usually, the roughness of the S side of an electrolytic copper foil depends on the roughness of the surface of the titanium drum. Therefore, the roughness of the S side of the electrolytic copper foil is relatively consistent, whereas the roughness of the M side can be controlled by adjusting the conditions of the copper sulfate electrolyte.
The current copper sulfate electrolytes for producing electrolytic copper foils for use in lithium ion secondary batteries can be mainly classified into two major categories, and one of which is the so-called additives-containing system, i.e., to a copper sulfate electrolyte, adding organic additives such as gelatin, hydroxyethyl cellulose (HEC) or polyethylene glycol (PEG), capable of inhibiting electrodeposition of copper ions, and sulfur-containing compounds such as sodium 3-mercaptopropane sulphonate (MPS) and bis-(3-sodiumsulfopropyl disulfide (SPS), capable of refining crystalline particles. As such, the roughness of the M side of the electrolytic copper foil can be lowered, and thereby obtaining an electrolytic copper foil with double-sided gloss and having a structure containing fine crystalline particles. The electrolytic copper foil produced by this type of additives-containing electrolyte system has typically a tensile strength of less than 40 kg/mm2. The other category is the so-called non-additives-containing system, i.e., no addition of any organic additives to a copper sulfate electrolyte. This type of non-additives-containing system is contrary to the additives-containing system. The lower the total content of the organics in the copper sulfate electrolyte, the higher the likelihood of obtaining a glossy electrolytic copper foil having low roughness at the M side and no abnormal protruded particles on the surface. Although no organic additives are added to the copper sulfate electrolyte obtained from the non-additives-containing system, the copper raw material used in the copper sulfate electrolyte are mainly derived from commercially available recycled copper wires. The surfaces of the copper wires contain grease or other organic substances, such that when the copper wires are dissolved in sulfuric acid, the electrolyte for producing an electrolytic copper foil would be filled with impurities like grease or organic impurities. The higher the content of the organic impurities, the higher the likelihood of generating an electrolytic copper foil having numerous abnormal protruded particles on the M side. Hence, no electrolytic copper foil having double-sided gloss is obtained.
Moreover, when the M side of an electrolytic copper foil has numerous abnormal protruded particles, the subsequent applications in the manufacture of electrolytic copper foil are usually problematic. For example, during a copper roughening treatment, the abnormal protruded particles on the M side easily induce point discharging, which cause the copper roughening particles to abnormally concentrate. Subsequently, when the copper clad laminate was formed by pressing the electrolytic copper foil, the residual copper which was formed due to incomplete etching can easily cause a short circuit. As a result, the yields of the downstream products are poor.
In order to reduce the impact of organic impurities on the M side and physical properties of the electrolytic copper foil generated by the non-additives-containing system, Japanese Patent number 3850155 and 2850321 of Nippon Denkai Ltd. disclose a method for removing organic impurities from a copper sulfate electrolyte. In the method, copper wires are pretreated prior to dissolution, by burning the surfaces of the copper wires under a temperature of from 600 to 900° C. for 30 to 60 minutes, and washing the surfaces of the copper wires by 100 g/L of an aqueous sulfuric acid solution to remove the organic impurities from the surfaces of the copper wires. On the other hand, an ozone-generating device is further used to degrade grease or organic impurities in the copper sulfate electrolyte obtained from the pretreated copper wires above, and using an activated carbon filtration device to remove the degraded products by adsorption. However, although the method can be used to effectively obtain a clean copper sulfate electrolyte, burning copper wires at a high temperature consumes a large amount of energy. Further, even though the surfaces of copper wires can be washed by a sulfuric acid aqueous solution to remove organic impurities, a small amount of copper can be similarly removed to cause copper loss. In addition, as ozone is gas, it does not retain in the copper sulfate electrolyte easily. Thus, the effectiveness of further degrading organic impurities using ozone is poor. Also, high concentrations of ozone brings hazard and safety concerns to human bodies.
Accordingly, the industry urgently needs to develop an electrolytic copper foil which is suitable for use in a lithium ion secondary battery having a high tensile strength, a high elongation rate after a heat treatment, low roughness at the M side, and an extremely small difference in roughness between the S side and the M side. Yet, the manufacture process of the electrolytic copper foil is simple and free of safety concerns without increasing the complexity of electrolytes.